Animals can forge bonds across species boundaries if the need for social contact pre-empts their normal biological imperatives. A cat raised with dogs doesn’t know it’s a cat, the logic goes.

Marc Bekoff, professor emeritus of ecology and evolutionary biology at the University of Colorado, who has written several books on animal emotions, believes wholeheartedly in such bonds. He says , “I think the choices animals make in cross-species relationships are the same as they’d make in same-species relationships. Some dogs don’t like every other dog. Animals are very selective about the other individuals who they let into their lives.” Even predators and prey (including his dog and a bunny) can form relationships—which as he points out requires “incredible trust” from the prey animal.

Strong attachments often arise in captive animals, says Bonnie Beaver of Texas A&M’s College of Veterinary Medicine. “Two very stressed individuals may lean on each other for comfort.” In most cases, cross-species friendships are forged most strongly when animals are young. But in captivity even an older captured animal might seek out a friend, including a member of another species.

It's clear that nonhuman animals (animals) form deep and enduring relationships with members of their own species that can be called love. Also, and rather obviously, other animals develop and maintain very close relationships with us. And, as time goes on, we're learning more about relationships called "odd couples" or "unlikely friendships (link is external)" because they involve members of different nonhuman species forming extremely close and improbable friendly relationships with one another. (Source)